In general, nonlinear optical effect means a nonlinear optical response proportional to the square, cube or higher power of the applied photoelectric field. As the second order nonlinear optical effects proportional to the square of the applied photoelectric field, second harmonic generation (SHG), optical rectification, photo-refractive effect, Pockels effect, parametric amplification, parametric oscillation, light sum frequency mixing, and light difference frequency mixing are known. As the third order nonlinear optical effects proportional to the cube of the applied photoelectric field, third harmonic generation (THG), optical Kerr effect, self-induced refractive index change and two-photon absorption are exemplified.
As the nonlinear optical materials exhibiting these nonlinear optical effects, a variety of inorganic materials have been found until now. However, it has been very difficult to use inorganic materials in practice for the reason that what is called molecular design to optimize desired nonlinear optical properties or various physical properties necessary to manufacture a device is difficult. On the other hand, organic compounds are not only capable of optimization of desired nonlinear optical properties by molecular design but also capable of control of other various physical properties and the possibility of practical use is high, so that organic materials are attracting public attention as promising nonlinear optical materials.
In recent years, of the nonlinear optical properties of organic compounds, third order nonlinear optical effect, in particular, non-resonant two-photon absorption is becoming the object of public attention. Two-photon absorption is a phenomenon such that a compound is excited by the absorption of two photons simultaneously. The case where two-photon absorption occurs in an energy region where (linear) absorption band of a compound is not present is called non-resonant two-photon absorption. In the following description, “two-photon absorption” means “non-resonant two-photon absorption” even when not especially indicated. Further, “simultaneous two-photon absorption” is sometimes referred to as merely “two-photon absorption” by omitting “simultaneous”.
The efficiency of non-resonant two-photon absorption is proportional to the square of photoelectric field applied (quadratic dependency of two-photon absorption). Accordingly, when a laser is irradiated on a two-dimensional plane, two-photon absorption occurs only at the position of high electric field intensity of the center part of laser spot, and two-photon absorption does not occur at all at the peripheral part of weak electric field intensity. On the other hand, in a three-dimensional space, two-photon absorption occurs only in a region having large electric field intensity at the focus where laser rays are converged through a lens, and two-photon absorption does not take place at all in a region being off the focus for the reason that the electric field intensity is weak. As compared with the linear absorption wherein excitation occurs at all the positions in proportion to the intensity of photoelectric field applied, spatial resolution is extraordinarily improved in the non-resonant two-photon absorption, since excitation takes place at only one point in the space due to the quadratic dependency.
In general, in the case of inducing non-resonant two-photon absorption, a short pulsed laser in a near infrared region having no absorption which is on the side longer than the wavelength region where the (linear) absorption band of a compound is present is used in many cases. Since a near infrared ray in what is called a transparent region is used in non-resonant two-photon absorption, an excited light can reach the inside of a sample without being absorbed or scattered, and one point inside the sample can be excited with extremely high spatial resolution due to quadratic dependency of non-resonant two-photon absorption.
Until now, the present applicant has applied for various patents concerning two-photon-sensitizing type three-dimensional recording materials using a compound inducing non-resonant two-photon absorption. These recording materials are recording materials containing at least (1) a two-photon absorption compound (a two-photon sensitizer), and (2) a refractive index modulating material or a fluorescence intensity modulating material, wherein compound (1) efficiently performs two-photon absorption and the acquired photo-energy is delivered to material (2) by means of photo-inductive electron transfer or energy transfer, and recording is conducted by modulating the refractive index or fluorescence intensity of material (2). By using non-resonant two-photon absorption in the light absorption process not one-photon absorption used in ordinary photo-recording, it becomes possible to write recording pits on ordinary position in the inside of the recording material with three-dimensional spatial resolution.
For example, JP-A-2007-87532 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”) discloses a technique using, as refractive index modulating material or fluorescence intensity modulating material (2), a material capable of modulating a refractive index by color development of a dye, or a material capable of modulating fluorescence by changing from non-fluorescence to fluorescent emission or from fluorescent emission to non-fluorescence (a material capable of modulating a refractive index or fluorescence by color development of a dye or a fluorescent dye). Further, JP-A-2005-320502 discloses a technique using, as refractive index modulating material or fluorescence intensity modulating material (2), a material capable of amplification of recording by forming a seed (a latent image nucleus) of extremely slightly color development of a dye or fluorescence changed, and then light-irradiating or heating (refractive index/fluorescence modulation, a latent image amplification system, a material forming a latent image capable of refractive index/fluorescence modulation by color development of a dye). JP-A-2005-29725 discloses a technique of using, as refractive index modulating material (2), a material capable of modulating a refractive index by forming a polymer by polymerization (a material capable of refractive index modulation by polymerization). Furthermore, JP-A-2005-97538 discloses a technique using, as the refractive index modulating material, a material of forming an extremely minute polymerization latent image nucleus and then actuating polymerization (refractive index modulation, a latent image polymerization system, a material forming a latent image capable of refractive index modulation by polymerization).
Two-photon sensitizing type three-dimensional recording materials in all of JP-A-2007-87532, JP-A-2005-320502, JP-A-2005-29725, JP-A-2005-97538 use, as the two-photon absorption compounds (two-photon sensitizers) (1), compounds actuating two-photon absorption with a light of 700 nm or more. However, in recent years, further various demands have been made. Above all, for obtaining higher recording density, a technique capable of non-resonant two-photon absorption recording by using a recording light in a wavelength region shorter than 700 nm has been required to form smaller pits in a recording material.
For satisfying such a demand, JP-A-2010-108588 discloses a two-photon absorption recording material capable of non-resonant two-photon absorption recording with a recording light in a wavelength region shorter than 700 nm and having sufficient recording and readout properties. JP-A-2010-108588 further discloses a polyphenyl compound having high two-photon absorption ability in the short wavelength region which is usable therein.